How long have you been in your IVA? You may find that creditors add interest and charges back on if you allow it to fail.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
I feel like yo are really asking about how to change your IP firm, rather than pull out of your IVA, but I'm afraid this is not possible.
You can fail your IVA but you need to take great care as this will put you right back to square one again in dealing with your debts. It is also possible that your IP will have to make you Bankrupt, depending on the terms of your IVA.
The best way forward will be to send them a written compliant via registered post (and copied by email) asking for your queries to be answered.
Are you sure you need to pull out because of reviews? My IVA company was - at times - appalling, they don't exist any more but they did their job at the end of the day. I stuck mine out because of the time I had already invested, not because I liked them, I knew their were better firms but the end date is the end date, failing it will only put this back if you have to start again.